Instructables

DIY WiFi Helical Antenna impedance Problems

First off, I live in the states, but I absolutely hate the imperial system of measurements. I will try to use the metric system as much as possible.

I have been attempting to follow hanzablast's Helical WiFi Antenna, but have run into a few snags. I have most of the parts and have a few problems concerning it.

1. Instead of a Type N connector my USB card has a RP-SMA connector.
Do I have to do anything special (buy a special panel mount/ reverse the connections along the wire) because it has reverse polarity or does it not matter?

Edit: I feel foolish, Doing a quick wiki search on Reverse Polarity SMA I found out what it means. Turns out WiFi companies reverse the gender of the inner pin in their connectors. So either I have to find a RP-SMA panel mount jack, or buy a RP-SMA Male to N Male Adapter and a N Female Panel Mount. The latter of the two seems more efficient.

2. I understand that having too small of wire can have a detrimental effect on efficiency, but what about using a much larger wire (instead of AWG 16 gauge wire using AWG 4 gauge wire)?

Edit: Answered By NachoMahma

3. I am having trouble locating a SMA right angle 4-hole solder point panel jack mount other than in bulk buys. Does anyone know where to purchase one separately?

Edit: Answered By NachoMahma, but has become a null point.

4. I am having a problem with the impedance matching. I have seen several different ways to connect the antenna (142.68ohm impedance) to the SMA Jack (50ohm impedance). I've seen half and quarter circumference turns, quarter wavelengths turns, and triangular strips of copper. Which is best?

Edit: I did the math and read my friend's ARRL handbook. Supposedly, a 1/4 wavelength turn of a metal strip with an impedance of 84.463 ohm is what I need. Using TraceSim I figured that I can use part of the copper plate with a size of 12.79033mm in width, 30.775mm long (1/4 a 2.437GHz wavelength), and 0.406mm (.016") thickness placed 8.03mm (1/4 distance between coils) above the reflector plate will give me the exact impedance I need.

NachoMahma6 years ago
. If no one comes up an answer on the impedance, try American Radio Relay League. Look in the section on helical antennae.
. I talked to an EE today, and he said with a low-power transmitter like WiFi, an impedance mismatch shouldn't be a big deal. You won't get optimum performance (ie, range and bandwidth), but it shouldn't damage the final output as can happen with higher power radios. . I'd try it without impedance matching and, if it does what you want, run with it. You can always tweak it later.
NachoMahma6 years ago
1) Not sure what you mean by reverse polarity. As long as the center conductor doesn't get grounded, you should be OK with using SMA.
2) No problem using 4 AWG, but it really won't do much better than 16. If you were transmitting (much) higher power, it would matter. Your antenna wire doesn't need to be much larger than the center conductor of your coax.
3) Google SMA right angle 4-hole solder point panel jack
AllAgainstPaul (author)  NachoMahma6 years ago
Thanks for the insight on the wire gauge.
Many of the WiFi card makers now are using reverse polarity in there wires. The inside wire is reversed with the outside wire. They state it is to prevent us using antennas made for the incorrect wavelengths, but personally I think they did it so we have to buy more expensive wires and connectors.
. I'm still lost on the reverse polarity business. Radio signals don't have polarity - it changes several K/M/G times a second. As long as all the "polarities" are reversed (ie, the center conductor is not grounded), it shouldn't matter.
AllAgainstPaul (author)  NachoMahma6 years ago
Never mind what I was asking about reverse polarity, I was totally misinterpreting what I was reading.